Transformer



March 24, 1942. c, ss 2,277,499

TRANSFORMER Filed May 26, 19:59 2 Sheets-Sheet 1 I Fig.1

' INVENTOR. J0/1fl/V c. a. m/un ATTORNEY.

March 24, 1942. J. c. B. MISSEL Llllllllll INVENTOR. .m/mlvpamlmz'z.

ATTORNEY.

ages of the amplifying 'tubes.

Patented Mar. 24,

UNITED STATES PATENT OFFICE Johan Cornelis Bastiaan Missel,

Eindhoven,

Netherlands, assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application May 26, 1939, Serial No. 275,782 In Germany June 3,1938

1 Claim.

themselves as the so-called hum of the mains,.'

can be eliminated, for example, by electrostatically screening from one another the primary winding of the supply transformer and'the secondary winding which provides the anode 'volt- For electrostatic screening it has previously been proposed to arrange between the above-mentioned windings the winding which is connected to the heating circuits of the amplifying tubes.

'The applicant'has stated that with a superheterodyne receiver supplied byalternating current there occurs further interference which is due, for example, to the higher harmonics of the ing tube there occur resonance phenomena which make the higher harmonics present in a wide frequency range very prominent. I

The invention is based on the recognition that undesired disturbances reach the apparatus due to capacitative coupling of the intermediate-frequency amplifier and the mains-connection device and originate from the higher harmonics of the intermediate frequency which occur in theintermediate-frequency amplifier. If the anode vol tage winding of the supply transformer and the primary winding of this transformer are electrostatically screened from one another, the

hishe'rharmonics of the intermediate-frequency are substantially capacitatively transmitted to the heating circuits of the amplifying tubes and to the corresponding secondary winding and from the latter to the primary circuit of the .supply transformer and to the input circuit of the receiver and they consequently occur if the frequency of any one'of the higher harmonicssubstantially corresponds to the received frequency.

Since the transmission of the higher harmonics of the intermediate frequency is substantially effected capacitatively, the interference phenomena are, particularly manifest in receivers in which a high intermediate frequency (e. g. 450 kilocycles per second) is utilised.

According to the invention, the said interference' is eliminated by electrostatically screening from one another not only the primary winding and the secondary winding which provides the anode voltage but also the secondary winding connected to the heating circuits of the amplifying tubes and the primary winding.

For screening a metal foil, e. g. a copper foil, may be utilised in the manner known per so.

In order to effect satisfactory screening use must be made, however, of a fairly thick foil by which the construction of the transformer is made diflicult and which causes an increase of the dimensions of the transformer which is not negligible.

According to one convenient embodiment of the invention it is materially more advantageous to arrange the secondary winding provided for heating the cathode of the anode voltage rectifying tube in such manner that it forms an electrostatic screen between the primary winding on the one hand and the secondary winding providing the anode voltages and the secondary nected to the heating circuits or, if there are more than two heating windings, all the heating windings between the primary winding of the supply transformer and the secondary winding which furnished the anode voltage. The heating windings must of course always be arranged in such manner that the winding utilised for heating the rectifying tube, screens the primary windingfrom the other windings.

In asupply transformer whose windings are concentrically arranged above one another the screening winding should preferably be arranged, for the purpose of reducing the mutual capacity between the top ends of the coils, in such manher that it protrudes from the said ends.

The heating windings generally comprise only a small number of turns and require a smaller widthof winding than the other windings. For the electrostatic screening should preferably be utilised in uch cases a single-layer coil, part of which serves to heat the cathode of the anodevoltage rectifying tube.

The invention will be explained more fully with reference to the accompanying drawings.

Fig. 1 represents the wiring diagram of the high-frequency portion of an ordinary superheterodyne receiver. This portion comprises an aerial circuit I which is inductively coupled with an input band-pass filter 2, an oscillator circuit 3, a mixing tube 4 provided with a first intermediate-frequency band-pass filter 5 included in the anode circuit, an intermediate-frequency amplifying tube 6 and a second intermediatefrequency band-pass filter 1 whose output circuit is connected to a rectifying tube 20. For the supply of the amplifying tubes there is provided a mains-supply device of usual wiring which consists of a supply transformer 8 having a primary winding 9, a secondary winding ID for the supply of the anode voltage, a secondary winding H which is connected to the heating circuits of the amplifying tubes and a secondary winding it which serves to heat an anode voltage rectifying tube It and which is connected to a filter circuit l4 which serves to smooth the rectified current. Since the circuit-arrangement of the receiver and of the mains-supply device is substantially immaterial for the present invention it is not set out in detail.

Between the winding heating circuits of the amplifying tubes and the other elements of the arrangement there exists for high-frequency voltages a coupling of substantially capacitative character which is due to the wiring as well as to the internal tube capacities of the amplifying tubes 4 and 6 and of the rectifying tube 20. In Fig. 1 these capacities are indicated in dotted lines by condensers l5, Hand 2 I. Owing to these capacities, more particularly to the capacity 2|, a voltage which is due to the higher harmonics of the intermediate frequency is transmitted to those ends of the heating winding II which are not earthed. Due

to the capacity ll existing between the heating winding II and the rimary winding 9 (which capacity is indicated in dotted lines) the interfering voltages are transferred to the primary winding 9 whence they reach, generally by ca- I pacitative transmission IS, the input circuits I ll, connected to the I as faras possible without the transformer itself cies which become perceptible as an intzrference whistle noise. Particularly if the apparatus is not grounded or, as is frequently the case in practice, is not grounded in a faultless manner, i. e. that the resistance of the earth connection is not very small (shown in Fig. l as a resistance l9 represented by a dotted line), the interfering whistling noises become perceptible in a particularly unpleasant manner. According to the invention, these interfering frequencies are elimi nated by screening from the primary winding 9 not only the secondary winding l0 but also the heating winding l I.

Fig. 2 represents a cross-sectional view of a convenient form of construction of a supply transformer 20 in which the above-mentioned windings 9, l0, and II are screened from one another by the use of the .winding I3 provided for the heating of the rectifying tube. This winding itself covers, however, too small a wavelength and, in order to ensure satisfactory screening, it has been extended by dead windings :3.

In view of the capacity existing between the top ends of the coils it has been found to be advantageous to cause the screening winding to protrude from the other windings, which is in general possible without particular measures owing to the rigidity of the insulating intermediate layers 2| and of the turns themselves. In order to obtain a slight coupling between the secondary and primary windings it has proved to be advantageous to divide the heating winding H into two portions and to arrange the latter in the neighbourhood of the top ends. For screening the middle portions of the coils use may, if desired, be made as with the winding l3, l3, of a coil which is not utilised.

By utilising the transformer shown in Fig. 2 the above mentioned phenomena are suppressed being more expensive or occupying more room than an ordinary mains transformer.

What I claim is:

In a power transformer assembly, a primary winding and a plurality of secondary windings. one of said secondary windings forming an electro static shield between the other secondary winding and the primary winding, said windings being concentrically arranged about one another, said shielding winding being divided into two portions and arranged so that said two portions act to screen the two ends of the other secondary winding from the primary winding, there being provided a space between the adjacent ends of said two portions sufficient to leave the central portions of the other windings free.

JOHAN CORNELIS BASTIAAN MISSEL. 

